The present invention relates to rotor-shaft assemblies of the type used in exhaust gas driven turbochargers, and more particularly to the attachment of a ceramic rotor to a metal shaft assembly.
One means of improving the response time of a turbocharger is to reduce the moment of inertia of the rotating parts by constructing the parts of lighter material. Yet the material chosen must be able to withstand the harsh operating environment of the turbocharger. Since the compressor impeller does not see high temperatures in comparison to the turbine wheel, designers began to make the impellers of low weight aluminum alloy which can harmoniously exist in the turbocharger environment.
In the never ending quest for a lighter, economically feasible alternative to the relatively heavy tubine wheel, and one which could survive the high temperatures and gaseous environment of the turbine, the industry's focus turned to using ceramics as a substitute. Once the decision is made to use the lighter, ceramic turbine wheel, there are two alternatives design choices which must be considered; either construct only a ceramic turbine wheel or an integral ceramic shaft and turbine wheel. However, there are problems associated with either solution; the first requires a ceramic to metal joint and the second revolves around economics and durability of a ceramic shaft as compared to a metal shaft and the problems associated with attachment of the compressor impeller thereto.
Thereafter, most efforts have been focused on solving the problem of the ceramic to metal joint as evidenced by U.S. Pat. Nos. 4,063,850; 4,125,344 and 4,424,003 and German Pat. No. 2,734,747. However, none of these efforts have resulted in a reliable joint as evidenced by the fact that there is no commercially available or production model turbine wheel on the market, whether it be in turbochargers or any other turbomachinery. Several of these structures focus on shrink fitting the ceramic stub shaft of the turbine wheel within a metallic sleeve while others have concentrated on the use of an adhesive in order to bond the two materials together.
Utilization of the shrink fit method of attachment gives rise to a further problem: the reduction in the imposition of the high tensile stresses upon the ceramic stub shaft by the sudden discontinuity of contact between the sleeve member and ceramic rotor. This problem leads to the design feature of scheduling the compressive forces exerted by the sleeve onto the shaft by substantially tapering the thickness of the sleeve. This reduction in the thickness of the sleeve results in a reduction in the compressive stresses acting on the rotor and the tensile stresses imposed on the ceramic rotor at the point where contact between the sleeve and rotor ends. It is the tensile stresses which cause the propagation cracks in the ceramic material and can eventually lead to joint failure.
The high temperature, thermal cycling atmosphere of the turbocharger leads to the degradation and failure of the ceramic rotor-metal shaft joint. Failures occur because of several reasons; the metal sleeve radially expands by a greater degree than the ceramic rotor because it has a coefficient of expansion greater than a ceramic thereby loosening the joint; thermal cycling causes "ratcheting", i.e. the easing out of the ceramic stub from the sleeve; and in the case of adhesives, the breakdown of the adhesive in the high temperature environment.
In addition to the above problem of joint integrity, there exists a secondary problem of oil containment if the ceramic rotor or the ceramic to metal joint fails. Heretofore, none of the existing metal to ceramic joints have incorporated any means of preventing oil leakage into the turbine housing in the event of a joint failure.
According to the present invention, a ceramic rotor is attached to a metal shaft to form a rotor-shaft assembly. The rotor-shaft assembly includes a metal sleeve member having a generally coaxial bore formed therethrough. One end of the sleeve extends generally radially outward to form a hub portion which defines an annular surface area generally coaxial to the shaft. The sleeve hub portion includes an annular groove which is sized to generally mate with a piston ring located within the center housing near the turbine end of the turbocharger. The ceramic rotor includes a hub and plurality of blades periodically spaced about the circumference of the hub. The rotor further includes a stub shaft integral with and generally symmetrical about the axis of the hub. The stub shaft is cold press fitted within the end of the sleeve which defines the sleeve hub portion. In addition, the stub shaft has an annular groove therearound. Once the stub has been cold pressed into the sleeve member, a crimped groove, corresponding in location to the groove in the stub, is rolled into the sleeve member. The other end of the sleeve is then interference fitted or brazed onto the shaft in order to place the shaft in torque receiving relationship with the rotor.
It is an object of the present invention to provide a ceramic to metal joint for use within a turbocharger.
It is another object of the present invention to provide a sleeve member for joining a ceramic rotor to a metal shaft and including a portion of a seal between the center housing and the turbine housing.
It is another object of the present invention to provide a means for preventing lubricant from entering the turbine housing in the event of a joint failure or ceramic rotor failure.
It is a further object to provide a method of assemblying a ceramic rotor to a metal shaft.